Here Christian, bishop of Prussia, who had received from the Polish duke of Masovia a part of Kulmerland as a fief, had founded the knightly Order of Dobrzin, and was attempting with its aid to subdue the heathens of Prussia.
The security of the kingdom was sensibly promoted by the erection of a cordon of fortresses on its north-eastern borders, and a blow was given to foreign interference when Casimir succeeded in gaining dominant influence over the independent Polish principality of Masovia, which had hitherto gravitated between Bohemia and the Teutonic Order.
But Masovia to the north, and Great Poland to the north-west, refused to recognize the supremacy of Little Poland, while Silesia soon became completely germanized.
In the first year of the 13th century, the Knights of the Sword, one of the numerous orders of crusading military monks, had been founded in Livonia to "convert" the pagan Letts, and, in 1208, the still more powerful Teutonic order was invited by Duke Conrad of Masovia to settle in the district of Kulm (roughly corresponding to modern East Prussia) to protect his territories against the incursions of the savage Prussians, a race closely akin to the Lithuanians.
A second Tatar raid in 1259, less dangerous, perhaps, but certainly more ruinous, than the first invasion - for the principalities of Little Poland and Sandomir were systematically ravaged for three months - still further but Poland formed but a small portion of his vast domains, and Poland's interests were subordinated to the larger demands of an imperial policy which embraced half Europe within its orbit On the death of Louis there ensued an interregnum of two years marked by fierce civil wars, instigated by duke Ziemovit of Masovia, the northernmost province of Poland, the daughter of Louis the Great and the granddaughter of Wladislaus Lokietek, had an equal right, by inheritance, to the thrones of Hungary and Poland.